Experimental measurements and numerical analysis were carried out to study the effect of the cell geometry in resistivity determinations. The resistance of the diaphragm in cement paste and mortar samples was determined using impedance spectroscopy. Numerical simulations were performed using finite element method (FEM). Several surface ratios (geometrical diaphragm surface to electrolyte-diaphragm surface, S/S′) were investigated. The thickness of the diaphragm, L, was also considered. The experimental results show a significant decrease of the apparent resistivity when the ratio S/S′ increases. Similar trend was observed for increasing values of the L/S′ ratio. The numerical simulations can explain the experimental findings and also allow to formulate a general rule for the design of migration and diffusion experiments in porous materials.